In some engines, exhaust gas sensors, such as an air-fuel ratio sensor for sensing the air-fuel ratio of exhaust gas and an O2 sensor for sensing the oxygen concentration, are attached to exhaust passages such as exhaust pipes. In order for such an exhaust gas sensor to output a normal and accurate sensing signal, it is generally necessary to raise the temperature of the exhaust gas sensor to an activation temperature. In such a case, an exhaust gas sensor is heated by a separate heating device such as a heater, because it takes a long time to solely heat an exhaust gas sensor by the heat contained in exhaust gas and passing through the exhaust passages.
On the other hand, in such an engine as an exhaust gas sensor is heated by a distinct heating device such as a heater, when condensation (where water vapor within an exhaust pipe is cooled down by outside air, and condensed into water) occurs at the exhaust gas sensor positioned in an exhaust passage while the exhaust gas sensor is being heated, it is possible that a heat shock may be caused in the exhaust gas sensor resulting in undesirable failure such as through cracking of a critical element of the sensor.
Accordingly, in Japanese Patent No. 3636047 an exhaust pipe temperature is estimated. When the estimated exhaust pipe temperature is higher than or equal to a predetermined value, it is judged that the temperature in an exhaust pipe is not at a temperature at which condensation occurs, and electric power supply to a heater is started. Conversely, stopping the supply of electric power to the heater associated with an exhaust gas sensor when the temperature in the exhaust passage is at a temperature at which condensation occurs, minimizes the chance of sensor failure (e.g., cracking of a sensor element), increasing the durability of the exhaust gas sensor.
According the Japanese Patent No. 3636047 the threshold temperature below which condensation occurs within the exhaust pipe is set by experiment to a substantially constant value (about 52° C. to 54° C.).